This invention relates generally to gas turbine engines and more particularly to inlet ducts for such engines.
It is known to extract bleed air from an aircraft gas turbine engine to perform auxiliary functions such as flap blowing, boundary layer control, thrust vectoring, lift enhancement, and exhaust cooling in an aircraft. Such aircraft often require that the bleed air flow and pressure levels remain essentially constant, even though the engine thrust can vary over a band of about 20% to 100% of maximum, depending on the phase of flight.
One known method of supplying bleed air while maintaining thrust levels is to incorporate a “FLADE” stage (FLADE being an acronym for “fan on blade”) in a conventional fan of a turbofan engine. Each blade of the FLADE stage includes an outer fan blade extending from the tip of a conventional fan blade. The outer fan blades are disposed within an outer duct that surrounds the conventional fan duct.
In prior art designs, co-annular circular ducts are used to supply air from the airframe integration plane (“AIP”) of the engine to the FLADE stage inlet. In most cases, the FLADE fan is adjusted using variable inlet guide vanes (“IGVs”) to maintain flow rates much less than the design maximum when the primary fan is running at its design maximum thrust level. The difference in operation between these components drives a larger inlet diameter than would be required with a conventional turbofan engine of the same total airflow size. In situations where a new vehicle is being designed, this larger diameter translates into overall vehicle size and cost. In situations where a fixed installation/airframe exists, this larger diameter can prevent the utilization of an engine with a FLADE fan stage.
If a circular inlet annulus constraint is enforced, the only remaining parameters that can be used to effect a similar diameter improvement would be a reduction in primary fan radius ratio or increase in fan design specific airflow. An increase in fan specific flow would significantly and negatively impact fan performance. A reduction in primary fan radius ratio would require a significant advance over state-of-the-art mechanical designs which are already in use.
Accordingly, there is a need for a gas turbine engine inlet apparatus which accommodates a FLADE stage within conventional size limitations.